It is known in the art of pole manufacturing that the suitability of a pole for a given purpose depends upon the materials from which it is constructed. Pole designs have been restricted by the fact that selection of the material for construction previously required a tradeoff with respect to a number of certain key desirable characteristics of the pole. Among the key characteristics are strength, resilience, weight, length, durability, resistance to environmental conditions, and the ease of transportation and erection. Optimal pole design has been confounded because materials which provide superiority in one characteristic generally have corresponding disadvantages in other characteristics.
Perhaps the oldest known method in the art of pole construction is the use of wooden poles, such as those commonly used for telephone lines. However, many modern pole uses require longer lengths than are practical, or even possible, with wood. While shorter length poles constructed of wood are relatively inexpensive and easy to erect, wood poles become increasingly more expensive as the desired length increases. Furthermore, wood poles are highly susceptible to rot, insect infestation, and bird attack. Known methods of preventing these latter problems present their own difficulties in that the chemicals used to treat the wood may leach out into the surrounding soil, causing environmental hazards. Finally, optimal construction of wooden poles requires that the pole be of one piece of uncut wood. This creates difficulties in transporting and erecting long poles, and it obviously limits the maximum pole length to the height of available trees from which the poles are made.
Metal pole construction has also long been known in the art. However, metal poles also have disadvantages. Although relatively strong and capable of being constructed in sections for ease of transportation and erection, metal poles have limited durability in that they are susceptible to rusting and other chemical deterioration. This is primarily because the moisture, chemicals, and abuse that a typical pole receives at its base abrade any resistant coatings and lead to rapid rusting and deterioration of the metal structure. Metal poles experience an acute problem in locations near roadways, marine environments, industrial plants, and aggressive soils. For example, the salt used to prevent ice accumulation on the roads inevitably comes into contact with the pole, accelerating its deterioration. Other chemicals commonly spilled onto roadways can easily be splashed onto metal poles, accelerating the deterioration. Marine environments are also very aggressive and substantially limit the life of a metal pole.
Further, metal poles implanted directly into the ground or with closely surrounding vegetation are subjected to constant moisture accumulation both underground and within the first few feet of the base. While separately preparing a foundation onto which a metal pole may be secured addresses the underground deterioration, this does not solve the problem of salt and chemical splash, abuse, or moisture at the ground line vicinity of the pole. Further, such foundations built on site have the disadvantage of being of variable quality depending on the skill of the designer, the worker, and the actual soil conditions. Likewise, the necessity for the design and construction of a separate foundation structure adds significantly to the time and expense required to erect such a pole. Attachment to such foundation presents a critical structural weak point. Bolts used for attachment of the pole to the base are themselves subject to environmental degradation. Additionally, the bolts and mechanical fasteners represent a weak point because of the imposed fatigue loads and thus the potential for failure in shear or tension. The bolts may also pull out under stress if they are not adequately embedded in the foundation.
The use of concrete poles has also been known in the prior art. The strength and durability of concrete poles is superior to other materials. Concrete poles also solve the problem of susceptibility to roadside conditions and moisture. However the greater weight of concrete poles precludes the use of very long poles. The weight causes problems both for transportation and for ease of installation. Methods have been devised for transporting concrete poles to the construction site in sections to address the weight problem. See, for example, U.S. Pat. No. 5,285,614 issued to Fouad, which is hereby incorporated by reference, and which describes a splicing mechanism for concrete poles to address the weight and length restrictions. However, the greater weight of concrete poles has significantly impeded their widespread use.
A new improvement in the art is the use of hybrid pole construction, where the advantages of two different types of poles can be utilized. However, a hybrid pole approach has engendered its own set of problems, not the least of which is the method for securing the upper steel or other lightweight material pole to the concrete base pole and the method for manufacturing the pole in a manner that permits the pole to support high loads while reducing pole weight. The most efficient way to manufacture a strong concrete pole is by centrifugally casting the pole. The centrifugal action compacts the concrete mix, making it denser and thus stronger. The hollow core results in a lighter weight pole as well as saving on the cost of raw materials. The smooth, circular cross-section of the concrete base pole makes it easier to embed into the ground, is the most efficient shape for wind-loading minimization, and is the easiest to centrifugally cast. A lightweight, hollow steel or other lightweight material upper pole, on the other hand, generally is stronger and provides greater torsion resistance if it has a multisided cross-section than if it has a circular cross-section. So far, there has been no pole design that takes advantage of all the most favorable characteristics of both types of materials in a hybrid construction.
It is an object of one preferred embodiment of this invention to provide a centrifugally-cast concrete pole base that is reinforced, prestressed, and post-tensioned and that has two different cross-sectional shapes in order to permit it to readily receive a multi-sided upper pole section. This permits the mounting of much taller metal, fiberglass, or other material poles on the concrete pole base in one or more pole sections so as to reach the desired height while utilizing all the best design characteristics of each individual material.
It is also an object of the present invention to provide for taller poles that are economical to manufacture and install, in terms of time, labor, and materials.